The present invention relates to a polymer blend and the film of the polymer blend as a surface layer or coating. More particularly, the invention relates to a polymer blend which may be coextruded on one or both sides of packaging film to give a sealing surface(s) which exhibits surprisingly excellent hot tack and heat sealability suitable for vertical and horizontal form and fill film packaging.
This invention addresses the unique problems associated with sealing vertical and horizontal form and fill packages. Packaging may be accomplished by filling with food, medical solutions, etc. from a horizontal position where the item is inserted into the package from a side opening in the package. Often times it is desirable to fill a package from a vertical position. This is often desirable when filling a bag with a free-flowing composition or with several items which would render a horizontal filling process cumbersome.
The form and fill film packaging requires good "hot tack" and it is desirable to have a low heat seal initiation temperature and broad seal temperature range. Hot tack may be defined as the bonding strength between two pieces of polymer film at the point immediately after their having been joined together by application of heat and while the polymer is in the semimolten/solidifying state. The vertical form and fill film packaging procedure requires a simultaneous sealing of the bottom seam of the package and filling of the package while the package is in a vertical position. Thus, the package is sealed by applying heat and the seal must attain sufficient strength to contain the filled materials while still hot, so that the material or product does not fall out the bottom seal. Not only does the weight of the material or product contribute to this tendency for the bottom seal to release, but in addition the pressure of air used to assist in transport of the product may exert an additional pressure against the seal.
In addition to good hot tack and a low and broad heat seal initiation temperature range, (1) a low coefficient of friction, (2) a high seal strength, and (3) good compatibility with other packaging materials are also important.
Heretofore, films for vertical and horizontal form and fill packaging have been deficient in one or more of these desired properties. A commonly used seal layer such as propylene-ethylene copolymer melts at a high temperature relative to the main packaging film; i.e. it has a high and narrow seal temperature range. In this case packaging speeds are slower than desired, energy consumption is high and damage to the main package may occur if heat seal temperature is not very closely controlled. An alternative sealing material has been the metal salts of an ethylene acrylic acid copolymer which have a poor coefficient of friction, and tend to stick on the forming mandrel during shaping of the package. These materials also have poor compatibility with other materials in the package so that reclaimability is poor.